Golf ball manufacturers are constantly trying to achieve the perfect balance between feel and performance. The physical characteristics of a golf ball are determined by the combined properties of the core, any intermediate layers, and the cover. These, in turn, are determined by the chemical compositions of each layer. The composition of some balls will provide for increased distance. Other compositions provide for improved spin.
For example, golf ball covers formed from balata allow players to achieve spin rates sufficient to control ball direction and distance, particularly on shorter shots, but balata covers are easily damaged. In contrast, golf balls covers formed from ionomer resins provide higher durability and overall distance, but the spin and feel are inferior compared to balata covered balls. And, unlike ionomer-covered golf balls, polyurethane covered golf balls can be formulated to possess the soft “feel” of balata covered golf balls, however, golf ball covers made from polyurethane have not, to date, fully matched ionomer-covered golf balls with respect to resilience or the rebound of the golf ball cover. In addition, polyurethane-covered golf balls are generally susceptible to yellowing (due to the aromatic nature of the material) and moisture absorption. While paints and moisture barrier layers may be used to counteract against the yellowing and moisture absorption in polyurethane-covered balls, the resilience and rebound are more difficult to design around.
Polyurea materials have recently come onto the scene for golf ball layer materials due to the improved resilience and adherence to layers formed of different materials. In addition, because polyurea-based compositions may be formed from aliphatic materials, the yellowing of aromatic polyurethane-covered golf balls is typically not an issue for aliphatic polyurea-covered golf balls. Similar to polyurethane, however, because a polyurea golf ball cover is generally softer than a thermoplastic ionomer golf ball cover, the shear (cut) resistance does not compare to an ionomer-covered golf ball.
Conventional polyurea compositions have several characteristics that are undesirable for golf equipment applications including uncontrollable reaction rates, non-homogenous mixtures, poor adhesion, shrinkage, and non-optimal chemical resistance. For example, the reaction times for conventional polyurea compositions are very fast, i.e., an aliphatic isocyanate and an aliphatic amine may react and gel in about 5 seconds, which make it difficult to control the formation of the composition. In addition, several reactions may take place in a polyurea composition, which result in a non-homogenous mixture. For instance, a first reaction may take place between the highly reactive components followed by subsequent reactions between the less reactive components. The non-homogenous nature may affect the finish, properties, and consistency of the resultant composition.
The adhesion properties of a conventional polyurea composition are also less than desirable. In fact, fast reactions between the amine and isocyanate do not allow adequate time for the polyurea to penetrate and adhere to a substrate. Furthermore, conventional polyurea compositions have a shrinkage rate of higher than 1 percent when cured. As such, when a golf ball layer is formed from a conventional polyurea composition, the desired dimensions may not be realized. Finally, conventional polyurea compositions are susceptible to strong solvents, e.g., acetone, toluene, xylene, low pH acids, and high pH caustics.
Various additives have been used in an attempt to overcome the deficiencies of conventional polyureas formulations. However, no additive or component has yet been shown to overcome all of the above-referenced deficiencies.
Silicone materials have also been used in golf balls to purportedly increase the coefficient of restitution and/or durability based on their innate ability to provide materials having fairly high ultimate elongation. The use of such materials, however, has been primarily limited to interior layers of a golf ball. For example, U.S. Pat. No. 6,159,110 discloses the use of silicone polymers, silicone fluids, silicone elastomers, and silicone resins in interior golf ball layers. In addition, like conventional polyurea materials, conventional silicone materials have several characteristics that are undesirable, including low-moderate tensile strengths. Furthermore, to use silicone elastomers in the manufacture of golf balls requires covalent crosslinking because linear or branched silicone (polydimethylsiloxane) (PSX) homopolymers are viscous liquids or millable gums at room temperature.
Regardless of how the cross vulcanization is effected, the resulting thermoset silicone cannot be re-dissolved or re-melted, which severely reduces the number of options for post-fabrication operations For example, thermal forming, radio frequency welding, heat sealing and solvent bonding are essentially unavailable when working with conventional silicone elastomers. Once formed, however, the infinite network provides the polymer its rubber elasticity and characteristic physical-mechanical properties.
Epoxy resins have also been used in the production of golf balls. Generally, epoxy resins are good adhesives and can be formulated with different materials or other epoxy resins to achieve specific performance features. For example, U.S. Pat. No. 5,908,358 discloses polyurethane compositions cured with epoxy-based curing agents for improved shear resistance. Typical epoxy resin formulas do not have particularly good UV resistance. In addition, since the viscosity of epoxy is much higher than most polyester resins, typical epoxy resin formulas are slow to cure and require a post-cure (elevated temperature). Furthermore, brittleness and decreased flexibility can occur once cured.
It would be advantageous to incorporate the favorable properties of each individual material into a composition for use in golf balls so that the strengths of each material can be maximized and the weaknesses minimized. For example, there is a need in the art for a polyurea composition with a controllable reaction rate, homogenous properties, decreased shrinkage, and increased chemical resistance. In addition, there is a need in the art for a golf ball with at least one layer formed from a thermoplastic silicone-urethane. The present invention seeks to address these needs.